Re: [PATCH v3 1/2] rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs

Alice Ryhl
2025-12-11T01:21:35Z

On Tue, Dec 09, 2025 at 01:14:08PM +0100, Benno Lossin wrote:
> On Tue Dec 9, 2025 at 3:57 AM CET, Alexandre Courbot wrote:
> > This is useful when using types that may or may not be empty in generic
> > code relying on these traits. It is also safe because technically a
> > no-op.
> >
> > Reviewed-by: Alistair Popple <apopple@nvidia.com>
> > Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> > ---
> > rust/kernel/transmute.rs | 8 ++++++++
> > 1 file changed, 8 insertions(+)
> >
> > diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> > index be5dbf3829e2..a888a312e7ff 100644
> > --- a/rust/kernel/transmute.rs
> > +++ b/rust/kernel/transmute.rs
> > @@ -170,6 +170,10 @@ macro_rules! impl_frombytes {
> > }
> >
> > impl_frombytes! {
> > + // SAFETY: Inhabited ZSTs only have one possible bit pattern.
>
> Even inhabited ZSTs cannot just be conjured out of thin air, since there
> might be safety invariants on the creation of such a type. Now these
> two concrete ones do not have any, so this is fine, but should still be
> mentioned in the comment IMO.

I think we can say "it has no invariants".

Alice